Microbial Quantity Impacts Drosophila Nutrition, Development, and Lifespan

Abstract

In Drosophila, microbial association can promote development or extend life. We tested the impact of microbial association during malnutrition and show that microbial quantity is a predictor of fly longevity. Although all tested microbes, when abundantly provided, can rescue lifespan on low-protein diet, the effect of a single inoculation seems linked to the ability of that microbial strain to thrive under experimental conditions. Microbes, dead or alive, phenocopy dietary protein, and the calculated dependence on microbial protein content is similar to the protein requirements determined from fly feeding studies, suggesting that microbes enhance host protein nutrition by serving as protein-rich food. Microbes that enhance larval growth are also associated with the ability to better thrive on fly culture medium. Our results suggest an unanticipated range of microbial species that promote fly development and longevity and highlight microbial quantity as an important determinant of effects on physiology and lifespan during undernutrition.

Keywords: Microbiology; Microbiome; Nutrition in Life Cycle.

Graphical abstract

Figure 1

Association with Microbes Modulates Adult Lifespan on Undernutrition Diet (A) Survival of axenic or monoxenic Canton-S male flies on 0.1% yeast extract (YE) malnutrition diet (axenic versus L. plantarum, p = 0.75, all other comparisons to axenic, p ≤ 0.03, log rank test). Axenic flies were associated as embryos with specific microbes. N = 59–64 flies. (B) Survival of axenic or monoxenic Canton-S male flies on 0.1% YE diet (axenic versus L. plantarum, p = 0.28, all other comparisons to axenic, p ≤ 1.0 × 10⁻⁶, log rank test). Axenic flies were associated with microbes 1 day after eclosion, except for the indicated trial with I. orientalis, which was inoculated 8 days after eclosion. N = 58–66 flies. See also Figure S1.

Figure 2

Microbial Growth in the Fly Environment Is Positively Correlated with Extended Life (A) Microbe numbers in the fly environment correlate with median lifespan on malnutrition diet (r = 0.75, p = 0.016, Spearman's correlation coefficient). Average microbe counts from fly enclosures (N = 3), including the food surface, collected on day 8 or 13 from two independent survival studies with adult-associated live microbes. (B) Microbial mass calculated from microbe counts (80 pg/cell for I. orientalis and S. cerevisiae, 1 pg/cell for A. indonesiensis and L. plantarum) correlates with median lifespan (r = 0.93, p = 0.00020, Spearman's correlation coefficient). (C) Methylparaben abolishes the benefit of microbes on lifespan. None of the survival curves differ significantly from that of axenic flies (p > 0.05 for each comparison, log rank test). Median lifespan of microbe-associated flies also does not differ from that of axenic animals (p > 0.05 for each comparison, Fisher's exact test). N = 60–62 flies. (D) Methylparaben eliminates I. orientalis, A. indonesiensis, and S. cerevisiae in the fly environment. Shown are average microbe counts (+ SD) from spent fly vials (N = 3) collected on days 13 and 24 (3–4 days after food change). N.D., not detected (in all replicates). Flies were maintained on 0.1% YE malnutrition diet with either our standard acid-based preservative or 0.3% methylparaben. See also Figure S2.

Figure 3

Food Change Interval Affects Lifespan and Microbial Abundance (A) Survival of axenic or adult-associated, monoxenic flies on 0.1% YE malnutrition diet with fresh food provided every 3–4 days (left) or daily (right) (axenic versus microbes, 3-4–day transfers: p ≤ 0.01 for each comparison; daily transfers: p > 0.05 for axenic versus A. indonesiensis or L. plantarum, p ≤ 0.0029 for axenic versus I. orientalis or S. cerevisiae, log rank test). N = 56–65 flies. (B) Average microbial counts (+SD) in adult flies (left) or spent vials (right) when flies were transferred to fresh vials every 3–4 days or daily (averaged over CFU counts from days 4, 8, 12, and 16 from the study shown in A). For each day sampled, CFU/fly was calculated from N = 6 flies each and CFU/vial was calculated from N = 3 enclosures each. N.D., not detected (in all replicates). CFU, colony-forming unit. See also Figure S3.

Figure 4

Microbes Extend Fly Lifespan in a Dose-Dependent Manner (A) Survival of axenic or microbe-associated flies on 0.1% YE malnutrition diet. For microbe-associated conditions, 5 × 10⁵ CFU of live I. orientalis or S. cerevisiae or 5 × 10⁷ CFU of live A. indonesiensis was supplied one time in early adulthood, or the indicated quantity of autoclaved (heat-killed [HK]) microbes were provided either one time (single dose) or at every food change (twice/week) throughout life (recurring). A single dose of HK microbes resulted in no change in survival (p > 0.05, log rank test). Both live and recurring doses of HK microbes resulted in significant lifespan extension compared with the axenic control (p ≤ 4.3 × 10⁻⁶ for each comparison, log rank test). N = 57–63 flies. (B) HK microbes supplied throughout life can extend lifespan on a malnutrition diet (0.1% YE) (axenic versus live or HK microbes; p ≤ 1.5 × 10⁻⁸, log rank test). Axenic flies were associated once with live microbes or continuously with HK microbes throughout life. The indicated CFU of HK microbes was supplied at every food change. N = 57–64 flies. (C) Relationship between lifespan extension and the estimated protein content of different doses of HK microbes. Lifespan extension is calculated as the change in median lifespan between flies receiving lifelong HK microbe supplementation and axenic controls. Estimated protein content from each dose of HK microbes is divided by the initial number of flies per enclosure and number of days between transfers to fresh food to show the protein content available per fly per day. N = 57–64 flies. (D) Effect of yeast extract (YE) concentration on lifespan. As YE is reduced from the highest concentration tested (5%), dietary restriction results in lifespan extension. From the point of maximal longevity (0.25–0.5% YE), further reduction in YE results in malnutrition and drastically reduced lifespan. Median lifespan is plotted with whiskers representing the interquartile range (first and third quartiles). Diets were composed of the indicated YE (%, w/v) in a base medium containing 5% sucrose and 8.6% cornmeal (all w/v). N = 64–158 axenic flies. CFU, colony-forming unit.

Figure 5

Microbes Provide Specific Nutritional Benefits under Different Malnutrition Diets (A) On a starvation diet (0YE/0S = 0% yeast extract +0% sucrose), lifelong supplementation of live or autoclaved (heat-killed [HK]) microbes, YE, or tryptone has limited impact on longevity (left: axenic versus 0.075% tryptone, p > 0.05, all other comparisons, p ≤ 0.033; middle: all comparisons to axenic, p ≤ 0.015; right: all comparisons to axenic, p ≤ 0.0002, log rank test). At every food change, live or HK microbes (3 × 10⁸ CFU/vial of I. orientalis or S. cerevisiae or 2.5 ×10¹⁰ CFU/vial of A. indonesiensis or L. plantarum), tryptone (equivalent to 0.075% or 0.75%, w/v, final), or YE (0.075% or 0.75%) were supplied. N = 59–64 flies. (B) On a sucrose-only diet (0YE/5S), only lifelong supplementation of YE or certain microbes extends lifespan maximally (left: all comparisons to axenic, p ≤ 0.037; middle: all comparisons to axenic, p ≤ 5.0 × 10⁻⁶; right: all comparisons to axenic, p ≤ 5.4 × 10⁻⁸, log rank test). N = 56–66 flies. (C) On a low yeast medium (0.1YE/5S), all additives supplied throughout life (tryptone, YE, or live or HK microbes) strongly extend lifespan (left: all comparisons, p < 1.0 × 10⁻¹⁰; middle: all comparisons, p < 1.0 × 10⁻¹⁰; right: all comparisons, p < 1.0 × 10⁻¹⁰, log rank test). N = 59–60 flies. All flies used were initially axenic. CFU, colony-forming unit.

Figure 6

Microbial Quantity Is Associated with Enhanced Larval Development (A) Larval size (mm²) on 0.5% YE diet 6 days after axenic treatment or monoxenic exposure to approximately 5.4 × 10⁶ CFU/well of L. plantarum^endoref, L. plantarum^WJL, A. pomorum^PQQ-ADH, or A. pomorum. Live microbes were provided one time immediately following axenic processing of embryos (**p ≤ 0.01, ****p ≤ 0.0001, Dunn's multiple comparisons test). Average of all larvae from N = 3 arenas per group is represented by a horizontal line placed between error bars representing ± SD; individual arena averages are shown in (B). N = 46–121 measured larvae per strain. (B) Average larval size (mm²) compared with CFU from larval food surfaces in N = 3 arenas containing the cohorts of larvae displayed in A (r = 0.88, p < 0.0001, Spearman's correlation coefficient). CFU, colony-forming unit. See also Figures S4 and S5.